The present invention refers to an apparatus for pneumatically conveying bulk material.
An apparatus of this kind is known from German patents Nos. DE-PS 23 05 030 and DE-PS 25 50 164 and includes a conveyor line for transporting the bulk material and a secondary line guiding a clean gas and being subdivided in sections which are separated from each other by shut-off valves. Each shut-off valve is controlled by a pressure transducer which compares the actual pressure in the conveyor line with a desired pressure and actuates the respective shut-off valve at a pressure drop in the conveyor line by cutting off the downstream part of the secondary line so as to allow clean gas to be introduced from the secondary line into the conveyor line.
Such an apparatus permits a transport of the material in a slow and careful manner within the conveyor line at considerably charge of the conveying air flow since the pressure drop occurring downstream of the clogged area or of the beginning clogged area in the conveyor line is detected, and additional clean gas is fed into the conveyor line from the secondary line to dissolve the excessive material accumulation. According to these German patents, the pressure in the secondary line matches the pressure in the conveyor line during normal undisturbed operation. In the simplest case, the beginning of the secondary line is connected with the beginning of the conveyor line while the end of the secondary line is connected to the end of the conveyor line.
In the German patent no. DE-PS 23 05 030, the shut-off valves are designed as three-way/two-way directional control valves, with the third connection communicating with the conveyor line. Moreover, each shut-off valve includes a pressure transducer which switches over the three-way/two-way directional control valve at a pressure drop within the conveyor line in such a manner that the secondary line is shut-off downstream and the connection to the conveyor line is opened. Shut-off valves of this type are required in a great number and are relatively complicated and, moreover, tend to malfunction after a certain operating period because dusty bulk material from the conveyor line penetrates into the shut-off valve and into the secondary line. Such penetration cannot be eliminated even when providing a filter.
The German patent no. DE-PS 25 50 164 attempts to avoid this drawback by providing instead of the three-way/two-way directional control valves simple shut-off valves which are structurally combined with a pressure transducer. For introducing clean gas into the conveyor line, the secondary line sections arranged between the considerably spaced shut-off valves are connected with the conveyor line via suitably spaced check valves. Although these check valves may prevent a penetration of bulk material from the conveyor line into the secondary line, they can open only when the pressure difference between the secondary line and the conveyor line is sufficient to overcome the force of the valve spring. Moreover, since the pressure in the secondary line is adjusted to the pressure prevailing in the conveyor line during normal operation, the pressure available for feeding clean gas decreases in the secondary line from its beginning to its end.
The German patent no. DE-PS 33 23 739 and the German publication DE-OS 34 35 907 propose to avoid this drawback by providing a secondary line in which the pressure of the clean gas remains essentially constant and which is connected with the conveyor line via differential pressure controlled valves. The feeding of clean gas into the conveyor line is provided in dependence on the increase of the pressure difference between adjoining valves which are connected via respective lines.
These apparatuses thus have essentially the same drawback as the previously mentioned German patent no. DE-PS 23 05 030 and require complicated and specially designed valves which must be spaced from each other by a length corresponding to the length of the clogged area in order to ensure a dissolution of the clogged area, and thus must be provided in a great number.